1. Field of the Invention
The subject-matter of the present invention is a sintered shaped body consisting of a matrix material that contains an aluminium oxide/chromium oxide mixed crystal and which is in situ reinforced with platelets.
2. Description of Related Art
The use of an oxide ceramic material for pressing-tools for shaping components of glass or ceramic material containing glass is specified in DE-A-36 08 854. In addition to cubic and tetragonal zirconium dioxide, aluminium oxide, chromium oxide, spinel and an Al-Cr-mixed oxide (AlCr2O3) that is not defined with regard to its quantitative composition are also specified as matrix materials. The individual proposals for the matrix components enjoy equality in this connection so there is not offered any teaching for the selection of a particular matrix component or for the proportion of the quantity of, for example, zirconium dioxide to be incorporated in the matrix. Stabilizing oxides, such as, for example, yttrium oxide (Y2O3) in a quantity of 3.5 to 12, preferably 8 to 10, or magnesium oxide (MgO) in a quantity of 6.0 to 16, preferably 8 to 14 mol %, and cerium oxide (CeO2) in a quantity of 3.5 to 12 mol %, preferably 8 to 10 mol %, relative can also be present in addition to the components mentioned above. A size between 5 and 5000 nm, corresponding to 0.005 to 5 xcexcm, is mentioned as the particle size for the particles incorporated in a polycrystalline matrix.
A further proposal for a so-called xe2x80x9cconversion-reinforcedxe2x80x9d ceramic composition, in which a finely distributed solid solution of ZrO2xe2x80x94HfO2 in a solid solution of either aluminium oxide, containing chromium oxide, or mullite, containing chromium oxide, is specified, is found in WO 85/01936 and in that case is put forward for high-temperature fields of application, such as, for example, for diesel engines and gas turbines. The proportion of chromium oxide considered between 3 and 30 mol %, in particular a proportion of 20 mol % chromium oxide, cooperating with a proportion of 10 to 20 mol % hafnium dioxide, is to be used to improve the hardness and to set a low level of thermal conductivity. Rising proportions of chromium oxide and hafnium dioxide result in a decrease in the thermal conductivity. Noticeable increases in hardness are first found when there are comparatively high concentrations of chromium oxidexe2x80x94approximately 20 mol %, relative to 20 mol % HfO2. An order of magnitude of 5 xcexcm is specified for the grain size of the incorporated ZrO2xe2x80x94HfO2 phase in the examples of this specification, and the fact that tetragonal modification is not obtained is attributed to the fact that there not been achieved any success in obtaining the dispersed ZrO2xe2x80x94HfO2 solid solution with the sufficient degree of fineness. No addition of stabilizing oxides is mentioned in this specification. The fracture-toughness values attained lie in the range between 5 and approximately 6.5 MPavm.
EP-A-199 459 relates to ceramic compositions with high levels of toughness and provides for the cooperation of zirconium dioxide, partly stabilized zirconium dioxide, solid solutions of zirconium dioxide/hafnium dioxide, solid solutions of partly stabilized zirconium dioxide/hafnium dioxide, partly stabilized hafnium dioxide and hafnium dioxide with mixtures of metal oxides, in particular yttrium niobium oxide (YNbO4) or yttrium tantalum oxide (YTaO4), with the yttrium ion of the mixed oxides even being replaced in part by a cation of a rare earth metal, for example La+3, Ce+4, Ce+3, Pr+2, Tm+3. According to a further variant of this specification, the ceramic alloy that is described, that is, for example ZrO2, can be mixed, whilst adding YNbO4 in a quantity of at least 5 % by volume, with, for example, xcex1-aluminium oxide or even Al2O3xe2x80x94Cr2O3, mullite or titanium carbide. The disadvantage of this known composition can be seen in the fact that, as a consequence of the mixed oxides containing Nb or Ta that are produced, a further grain-boundary develops with the ceramic products and a softening point sets in that is not yet at a sufficiently high level for many fields of application.
Similarly, U.S. Pat. No. 4,770,673 describes a ceramic cutting tool, 20 to 45% of which consists of a zirconium dioxide alloy, containing 1 to 4 mol % of a mixed metal oxide, and 55 to 80% by weight of which consists of a hard ceramic composition, with the mixed metal oxides consisting of the group of YNbO4, YTaO4, MNbO4, MTaO4 and mixtures thereof, and M consisting of a cation, which is provided for the substitution of the yttrium cation, and being selected from Mg+2, Ca+2, Sc+3 and rare earth metal ions, consisting of the group LA+3, Ce+4, Ce+3, Pr+3, Nd+3, Sm+3, Eu+3, Gd+3, Tb+3, Dy+3, HO+3, Er+3, Tm+3, Yb+3 and Lu+3 and mixtures thereof. In addition to aluminium oxide and, for example, sialon, SiC, Si3N4, as a hard ceramic material Al2O3xe2x80x94Cr2O3 also comes into consideration, in which a proportion of Cr2O3 of up to approximately 5 mol % is provided. Here again, as previously, there is the disadvantage that too low a softening range results in the ceramic material on account of the alloying constituents that are added to the ZrO2 in the form of the mixed oxides which contain niobium or tantalum.
U.S. Pat. No. 4,316,964 relates to a composition that is also taken into consideration for the production of cutting plates and which consists of 95-5% by volume aluminium oxide and 5-95% by volume zirconium dioxide with the addition of approximately 0.5-5.5 mol % yttrium oxide, 0.5 to 10 mol % cerium oxide, 0.4 to 4 mol % erbium oxide and 0.5 to 5 mol % lanthanum oxide, relative to zirconium dioxide.
A sintered shaped body that is also provided for use as a cutting plate in accordance with EP-A-282 879 consists of a matrix containing whiskers and, moreover, particles of, for example, silicon carbide, silicon nitride, sialon, aluminium oxide and zirconium dioxide. The whiskers can be made of the same materials as the particles. Zirconium dioxide is mentioned here as the matrix material in addition to mullite and aluminium oxide. Moreover, the sintered shaped body can also contain the usual sintering aids, such as, for example, the oxides of magnesium, chromium or yttrium. Of the rare earth oxides that are suitable those which are preferred are the oxides of lanthanum, samarium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. Fracture-toughness values of more than 10 MPamxc2xd are specified.
A ceramic material with very high levels of toughness and wear-resistance for use as a metal-removing cutting tool is known from DE-A-35 29 265. The composition provides, in addition to 20 to 50% by weight titanium carbide and 18 to 79.9% by weight aluminium oxide, 0.1 to 2% by weight of a sintering aid which is selected from the group: MgO, CaO, SiO2, ZrO2, NiO, Th2O3, AIN, TiO, TiO2, Cr2O3 and/or at least one oxide of the rare earths. Y2O3, Dy2O3, Er2O3, Ho2O3, Gd2O3 and/or Tb4O7 are mentioned as rare earth oxides. The sintering aids are used to prevent the grain growth in the case of the aluminium oxide and enter into combination with the latter, promoting the sintering process of the ceramic material.
A sintered body containing 40 to 99 mol % partly stabilized zirconium dioxide and 1 to 60 mol % aluminium oxide and, furthermore, as sintering aids, small quantities of the oxides of Mn, Fe, Co, Ni, Cu and Zn for the acceleration of the sintering process, is known from EP-A-214 291. The oxides of yttrium, magnesium, calcium or cerium are proposed for the purpose of setting a tetragonal phase proportion of 65 % or more. 1.3 to 4 mol % is mentioned as the quantity of yttrium oxide that is to be added and which can be replaced completely or partly by the other stabilizing oxides in a quantity of 0.01 to 12 mol %.
A zirconium dioxide having more than a 65% tetragonal phase and being contained in a high-density ceramic body, 60 to 99 mol % of which consists of aluminium oxide, is specified in EP-A-236 507. Less than 3 mol % Y2O3, less than 12 mol % MgO or CaO and less than 14 mol % CeO2, relative to the ceramic composition, are proposed for the stabilization of the zirconium oxide. In order to improve the sintering capacity and in order to suppress the grain growth and thus to attain a particularly high density, the material also contains transition metal oxides of Mn, Fe, Co, Ni, Cu and Zn, which can be added as such or as hydroxides, nitrates, chlorides inter alia to the starting composition. The disadvantage of this known material is that the maximum hardness of 1,750 kg/mm2 is not yet sufficient for many fields of application, in particular in the case of cutting tools for machining.
The addition of chromium oxide to aluminium oxide, where at least 10% by weight chromium oxide is used, has been proposed for the production of a refractory material in U.S. Pat. No. 4,823,359. Alternatively, a mixture consisting of aluminium oxide/zirconium dioxide can also be used instead of the aluminium oxide. The comparatively high porosity that is desired for refractory articles and a low level of fracture toughness can be inferred from the permitted size of the grains, up to 50 xcexcm, before sintering. The use of stabilizing oxides and the presence of the zirconium dioxide, possibly used in a specific modification thereof, are not mentioned. In addition, chromium oxide together with aluminium oxide and zirconium dioxide is used in accordance with U.S. Pat. No. 4,792,538 to produce refractory articles. The quantity of chromium oxide here is 5 to 25% by weight, with preferably 16% by weight being used. The porosity here lies in the range of approximately 14 to 15%; the addition of stabilizing oxides and the presence of zirconium dioxide in a specific modification thereof are not addressed.
WO 90/11980 relates to a ceramic material in which platelet-like grains of strontium aluminate in a molar ratio of SrO/Al2O3 between 0.02 and 0.2 are incorporated in a matrix of ZrO2, Al2O3 or a mixture of Al2O3 and ZrO2, consisting predominantly of ZrO2, (platelet-reinforcement in situ). The hardness values that are attained are comparatively low even in the case of comparatively high proportions of aluminium oxide.
The in situ platelet-reinforcement of oxidic materials with chromium oxide doped SrAl12O19 platelets is also described in EP 0 542 815.
The underlying object of the present invention has consisted in improving the known materials and making sintered shaped bodies available that have a high level of strength and in which good fracture toughness is combined with a simultaneously high level of hardness. The present invention provides a sintered shaped body which meets these requirements and which, in consequence of its spectrum of properties, has a comparatively high level of wear resistance. The sintered shaped body in accordance with the invention is suitable for use as a cutting tool, in particular as a cutting plate, especially as a cutting plate for machining cast and steel materials. The properties of the sintered shaped body in accordance with the invention also render possible its use in particular as a cutting plate for interrupted cutting. Furthermore, the sintered shaped body in accordance with the invention can be used in other tribological applications.